Galileoscope Optics Activity Guide

Transcription

1 Galileoscope Optics Activity Guide Robert T. Sparks and Stephen M. Pompea National Optical Astronomy Observatory Tucson, Arizona USA Version 1.1 NOAO is operated by the Association of Universities for Research in Astronomy (AURA), Inc. under cooperative agreement with the National Science Foundation

2 Galileoscope Activity Guide Table of Contents Galileoscope Introduction 3 Summary of Galileoscope Activity Guide 4 Light Through an Acrylic Block: A Demonstration 6 Kinesthetic Optics 8 Light Through a Convex Lens (A Demonstration) 10 Finding the Focal Length Using a Distant Object 12 Simple Magnifiers 16 Build a Refracting Telescope I 20 Build a Refracting Telescope II 24 Build a Galileoscope 27 Vendor Information 29 More Background for the Interested Educator 30 Common Misconceptions About Light 39 Glossary 42 Appendix: Education Standards 44 This work was supported by a grant from the National Science Foundation to the American Astronomical Society for coordination of the International Year of Astronomy 2009 and a grant from the NSF for the Hands-On Optics and the Astronomy from the Ground Up ISE Projects. NOAO is operated by the Association of Universities for Research in Astronomy (AURA), Inc. under cooperative agreement with the National Science Foundation. For Suggestions and Comments Please Contact: Dr. S. Pompea Manager of Science Education U.S. Project Director, International Year of Astronomy 2009 Past Project Director and Co-PI, Hands-On Optics, Co-PI Astronomy from the Ground Up NOAO, 950 N. Cherry Avenue, Tucson AZ spompea@noao.edu 2

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4 Introduction to the Galileoscope Four hundred years ago, Galileo turned his small telescope toward the heavens. His observations transformed our understanding of the universe. Galileo saw craters on the Moon, phases of Venus, moons orbiting Jupiter, and ears on Saturn which we now know to be rings. His systematic observations challenged conventional scientific knowledge of the day. The International Year of Astronomy celebrates Galileo s achievements. In order to help people appreciate what he accomplished, we have developed the Galileoscope. The Galileoscope is a low cost, high optical quality telescope similar in size to the ones used by Galileo to make his historic observations. The Galileoscope is capable of resolving craters on the Moon, the phases of Venus, the four Galilean Moons of Jupiter and the rings of Saturn. The Galileoscope also can be used to observe bright double stars, star clusters, nebular, and even some of the brighter galaxies. The Galileoscope is designed as an educational kit. The Galileoscope requires some simple assembly so you can see the elements that go into making a small refracting telescope. This educational guide goes deeper into the optical principles behind telescopes. You will see how light bends when it enters different substances. You will get to learn about how lenses form images and measure their focal lengths. You will use lenses as magnifying glasses to learn how eyepieces work. And you will build the Galileoscope and use it to observe the heavens. This guide contains a series of demonstrations and activities centered around how telescopes focus light. Each activity includes student learning goals, an equipment list, step-by-step directions, student worksheets, and background information for teachers. When you have finished these activities and have your own Galileoscope, you can go out and observe the night sky. You can find the Galileoscope Observing Guide at 4

5 Summary of Galileoscope Activity Guide About These Activities The activities in this manual were originally developed for the National Science Foundation sponsored Hands-On Optics program ( The activities were adapted for another NSF project: Astronomy From the Ground Up. ( led by the Astronomical Society of the Pacific. A kit of materials was developed to go with these activities which included all the equipment needed to do these activities. Both the Hands-On Optics and Astronomy from the Ground Up projects were aimed at informal science educators such as museum educators and afterschool program leaders. The materials have been used very successfully by classroom teachers. At the end of this manual is a list of vendors you can use to obtain materials to do these activities. Alternately, many of the activities can be done by substituting household items or using the lenses from the Galileoscope. See each individual activity for suggestions on what equipment to use. Note that the Galileoscope can be assembled, used for observations, and then disassembled to use the lens for various experiments. A summary of activities is given now. Light Through an Acrylic Block: A Demonstration: minutes Starting with a laser pointer shining perpendicular (at what is called the normal incidence angle) to an acrylic block, the teacher will slowly increase the incident angle. The students will observe that the path of the light changes as the incident angle increases. This demonstrates that light can be bent or refracted by a material. Light Passing Through a Convex Lens : A Demonstration : minutes When parallel light beams encounter an object such as a lens, its shape can cause different light rays to bend by different amounts. Students predict the path of the rays through an acrylic block and through a lens, then determine if they are correct by using a mister or chalk dust to expose the laser beams. Finding the Focal Length Using a Distant Object: minutes When looking at a brightly colored lamp on one side of the room, students will measure the focal length of a lens by forming an image of the light on a screen and measuring the distance between the lens and the screen. Simple Magnifiers: minutes In this activity, students will explore the magnifying properties of the lenses and notice the connection between how much the lens is curved and its ability to magnify. The students can also see how a juice bottle filled with water can bend light and magnify. 5

6 Build a Refracting Telescope I: minutes This is the first of several activities relating to refracting telescopes like the Galileoscope. Students will first determine how to arrange two lenses so that when they look though them they will see a magnified image of a distant object. Build a Refracting Telescope II: minutes Using the configuration of lenses that they found previously, students will create a magnified image of a distant object. By placing the velum screen in varying locations, students will determine the function of each lens in a basic refracting telescope. Build a Galileoscope: minutes The students in groups of two or three will build the refracting telescope from the kit. They will then look through the telescope at distant objects, making notes about their observations. 6

7 Light Through an Acrylic Block: A Demonstration Overview This demonstration introduces the concept of refraction. Refraction occurs when light changes its direction of travel when its speed changes as it moves from one medium to another. An example is when light travels from air to water or from air into an acrylic block. Students Will Learn In a uniform medium, light will travel in a straight path. When light hits a boundary between two different substances, such as air and water, the path it follows can change. TIP: You can use a clear plastic What You Need For the class: rectangular glass or plastic container filled with water instead of an acrylic block. 1 acrylic block Place a drop of milk in the container so you 1 laser pointer can see the laser beam. Getting Ready Clear a space on a large table so everyone can see. Set up the laser and the acrylic block. Dimming the room lights (if practical) may make it easier to observe the demonstration. GO: Light Through a Glass Block 1. Place an acrylic block on a table and have the students gather around. Shine a laser pointer so the beam hits the surface at a right angle. The beam of light should be perpendicular to the surface that it is shining through. Ask the students if they notice anything happening to the direction of the beam. 2. Now slowly turn the laser so it hits the surface at an oblique angle. Again ask the students if anything has happened to the direction of the beam. 3. Keep increasing the angle at which you are holding the laser. Ask the students what is happening to the amount that the beam is being bent as the angle changes. Top View 7

8 What s Really Happening Here When light travels from air to glass, the speed of light will decrease. This change in speed causes the light to change its direction of travel at the boundary. Light follows Fermat s Principle. Fermat s Principle states that light will take the path between two points that is the shortest in time (i.e., light follows the quickest path). We usually think light travels in a straight line. In this case, however, a straight line is not the quickest path through the block due to the fact that the material that makes up the block slows the light down. So the light beam bends to find the path that would take the least amount of time. Students can do the Kinesthetic Optics Activity to learn why light bends when it encounters a boundary where it slows down. Going Further 1. Examine the point where the laser leaves the block and returns to the air. What happens to the path of the beam at this point? You can use chalk dust or canned fog to make the beam more visible. 2. Get a clear plastic glass and partially fill it with water. Place a pencil in the glass and notice how it appears bent at the boundary between air and water. Have students explain what is happening using their knowledge of refraction. 3. Suppose a lifeguard is standing on the beach and sees a swimmer who needs help. The lifeguard wants to get to the swimmer as quickly as possible. What path should she take? How does this relate to refraction? 4. Have students get into groups and let them explore the refraction of light using other objects in the classroom, such as fish tanks or other large, clear objects. Please remind them of Laser Safety Rules. 5. Place a coin at the bottom of an opaque cup. Have the students back away from the cup. Tell them to stop as soon as they can t see the coin anymore (be sure they realize that they will not all stop at the same point due to the fact that they are different heights). Slowly pour water into the cup. Ask students to describe their observations and determine the cause of this effect. 8

9 Kinesthetic Optics Overview This activity allows students to pretend to be wave fronts of light (much like an ocean wave). As the students walk from air to another substance such as plastic where light slows down, they will observe how the light changes direction. Students Will Learn Light travels slower in substances such as plastic and glass than in air Light bends toward the normal if it slows down when it passes from one substance to another Light bends away from the normal if it speeds up when it passes from one substance to another What You Need For the class A large open space Getting Ready You will need a large open space where students can move about. You will need a line in the center of the area. If you are using a basketball court or athletic field, choose one of the lines on the court or field. You can also make a line in the center of the area using floor tape. Go: Kinesthetic Optics 1. Tell the students they are going to learn why light bends when it passes from air to plastic. Show the students the line and tell them that one side of the line represents air and the other side of the line represents plastic. 2. Tell the students that light is a wave, similar to a water wave. They are going to represent a wavefront. A wavefront can be though of as the crest or top of a wave for our purposes. 3. Have the students line up next to each other and lock arms. Have the students practice walking at the same speed so the line stays straight as they walk. Tell the students this is how light moves in air, As long as light does not move to a different substance, all parts of the wavefront move at the same speed. 4. Have the students line up in the air facing the plastic. Have the students line up in the air and lock arms. Make the wavefront parallel to the air-plastic boundary line. Tell the students that light moves slower in plastic than air. Ask them what they will do when the get to the air-plastic boundary. Make sure the students understand they will walk slower when they move into the plastic. Tell them to walk about half as fast in the plastic. Remind them that they will all walk at the same speed as everyone else. Practice this a few times. Ask the students if light changes direction when it encounters the plastic. 5. Now have the students line up in the air and lock arms. Have the students rotate the entire line so they will hit the plastic at an angle. Remind the students that they will slow down only when the cross the air-plastic boundary line. Remind students at the end of the line that they will keep walking fast until they hit the line, even though others may have slowed down earlier. Tell the students to keep walking after they are in the plastic but at the slower speed. 9

10 6. Ask the students if light changed direction when it hit the boundary. They should notice light changed direction. If your students are familiar with the term, you can tell them light bent toward the normal. Have the students practice this several times. 7. Tell the students they are going to look at what happens when light travels from plastic to air. Ask them how their speed will change when they encounter the boundary. Tell students that they will start walking slowly in the plastic and then speed up when they encounter the plastic-air boundary. 8. Have the students line up in the plastic and lock arms. Make the line parallel to the boundary. Have the students practice walking slowly in the plastic and speeding up when they encounter the air several times. The line should not change directions in this case. 9. Now have the students rotate the line so they will encounter the air at an angle. Ask the students what they think will happen. Remind the students not to speed up until the encounter the plastic-air boundary. 10. Ask the students what happened to the direction they were moving. They should notice they turned the opposite direction as before. If you students are familiar with the term, you can tell them that light bends away from the normal. Have the students practice moving from plastic to air several times. What s Really Happening Here This analogy uses the wave nature of light. Each part of the This series shows a view from above of the students walking across the air-plastic boundary. wavefront will travel at the same speed until in encounters a different substance. If the wavefront approaches a boundary at an angle, one part of the wavefront encounters the boundary first and slows down. The other parts of the wavefront keep traveling fast until they encounter the boundary. This causes the entire wavefront to rotate and change directions. If the wave slows down, it will bend toward the normal. If the wave speeds up, it will bend away from the normal. 10

11 Light Through a Convex Lens (Demonstration) Overview In this activity, students will see how we can use the property of refraction to focus parallel rays of light. Students will observe how a convex lens can cause parallel rays of light to converge. Students Will Learn A convex lens can cause parallel rays of light to converge. The point at which parallel light rays meet is called the focal point. The distance from the lens to the point where the light rays meet is called the focal length. What You Need For the class: 1 laser pointer 1 acrylic block 1 velum screen or a wall 1 large positive lens Tip: Use a container of water instead of an acrylic block as mentioned in the previous activity. You can use the objective lens of the Galileoscope instead of the large positive lens. An index card can be used instead of velum. For each student: Copy of STUDENT HANDOUT: Which Laser Produced Which Spot? Getting Ready 1. Clear a space on a table. Set the laser on the table by rotating its legs until they are perpendicular to the laser body. 2. Set up a velum screen several feet away from the lasers (or alternately, have the lasers project onto a wall or other vertical surface). 3. Make a holder for the large positive lens. Cut the bottom off a Styrofoam cup so that the lens will sit in the cup. You may want to cut small notches in the side of the cup to make the lens more stable. Lens holder made from a Styrofoam cup. 11

12 GO: Light Through a Convex Lens 1. Shine the laser through the block. Starting at the left side of the block, slowly slide the laser to the right. Have the students observe how the spot on the screen moves. 2. Now replace the glass block with the positive lens. Place the laser so the beam goes through the left side of the lens. Slowly move the laser to the right. Have the students observe the how the spot on the screen moves.. 3. You can use a plant mister to spray water to reveal the path of the beam to the students. You can also show the path of the beam by using an index card to trace where the beam goes on a sheet of paper. What s Really Happening Here A converging lens, sometimes called a positive lens (due to the fact that it has a positive focal length) is capable of focusing parallel light rays down to a single point, called the focal point. You will notice that a converging lens has a curved surface and is thick in the middle and thin at the edges. Light rays that pass through the center of the lens will not change direction. Light rays that hit away from the center of the lens have a different angle of incidence. Therefore, they are refracted and will change their direction of travel. The shape of a converging lens causes all incoming parallel light rays to converge to a single point. The converging lens can focus incoming light rays that are not parallel as well. These light rays will not converge at the focal point. For more information, see Finding the Focal Length Using a Distant Object: What s Really Happening Here Going Further Can students find other materials in the classroom that can be used to create a focal point. NOTE: This is a place in the module to make it clear to the students that the focal point of a lens is a property of the lens and it DOES NOT change. Images are formed where rays are in-focus, which is NOT necessarily where the focal point of the lens is. 12

13 Finding the Focal Length Using a Distant Object Overview In this activity, students will learn how to focus images of a distant object onto a screen using a converging lens. By measuring the distance from the screen to the lens, students will determine the focal lengths of the lenses. Students Will Learn Converging lenses can be used to project an image onto a screen. A single converging lens will produce an inverted image on the screen. When focusing a distant object on the screen, the focal length of the lens is equal to the distance from the lens to the screen. What You Need For each group of students: 1 20-cm focal length lens cm focal length lens 1 velum screen 3 Styrofoam cups ruler Tip: You can use the objective lens of the Galileoscope for the long focal length lens and assemble the eyepiece for the short focal length lens in this activity. The focal lengths will be different (50 cm and 2 cm respectively). Getting Ready 1. You will need to have an object for the students to focus to produce an image. There are several objects you can use. If you have a large window in the classroom, you can dim the room lights and students can focus the image of objects outside such as trees or cars. You can buy small neon lamps at stores such as Wal-Mart or Walgreens for $10-$20 that also work well. 2. Remove the lenses from their boxes, because the focal lengths are printed on them. When cleaning up, remember that the thicker lens has the shorter focal length. GO: Finding the Focal Length Using a Distant Object 1. Hand out the lenses, screens, and rulers to each group. Be sure to remove the lenses from their boxes since the focal length of each lens is printed on the box. 2. Have the students complete the worksheet, STUDENT HANDOUT: Finding the Focal Length Using a Distant Object. Assist the students as necessary. 13

14 What s Really Happening Here When you look at a very distant object (very distant means the distance to the object is very large when compared to the focal length of the lens) an image will form at the focal point. This image will be upside down and much smaller than the object. A camera works based on this principle. A converging lens focuses an image on the film. To see the mathematical description of this phenomenon, see More Background for the Interested Educator. Going Further Ask students to think about other optical devices that use a converging lens to focus an image on a screen. Ask students if they can think of a way to project an image onto a screen that is right side up. 14

15 STUDENT HANDOUT: Finding the Focal Length Using a Distant Object What You Need 1 thick convex lens (or Galileoscope eyepiece) 1 thin convex lens (or Galileoscope objective lens) 1 velum screen 3 Styrofoam cups ruler What To Do 1. Examine the two lenses. How are they similar? How are they different? 2. Create a lens holder. Cut a slit in the bottom of the Styrofoam cup, like in the picture at right. Insert the thick lens into the slit. Hold the lens in place with tape as shown in the picture at far right. Cut a slit in the bottom of another Styrofoam cup. Insert the velum screen into this slit. 3. Your instructor will point out an object to use. It may be a tree outside or a light in the room. Place the screen holder on the table. Move the lens closer to and farther away from the screen until you see an image of the object on your screen. Move the lens until you see a well focused image. a. Is the image right side up or upside down? Is it larger (magnified) or smaller than the original object? b. Measure the distance from the thick lens to the velum screen. Record this distance. What is this distance called? 4. Repeat the procedure for question 3 for the thin lens. a. Is the image right side up or upside down? Is it bigger (magnified) or smaller than the original object? THIS PAGE MAY BE PHOTOCOPIED FOR CLASSROOM OR WORKSHOP USE 15

16 b. Measure the distance from the thin lens to the velum screen. Record this distance. 5. The distances you measured in questions 3b and 4b are called focal length. Ask your teacher to tell you the focal length of the lenses. How do these numbers compare with what you measured? 6. Describe the differences between the shape of the long focal length lens and the short focal length lens. 7. Based on your observations, what can you conclude about the relationship between the focal length of a lens and the shape of a lens? 16

17 Simple Magnifiers Overview In the previous section, some fundamental properties of converging lenses were investigated. Students used a converging lens to project an image onto a screen. An image that can be projected onto a screen is called a real image. However, converging lenses can also produce images that cannot be projected onto a screen these are called virtual images. For a converging lens, the virtual image is right side up and larger than the object. A converging lens used in this way is commonly called a magnifying glass. Students Will Learn Converging lenses can be used to magnify an object. The amount of magnification is related to the focal length of the lens. The point at which an image flips is the focal point. What You Need Tip: You can use the objective lens of the For each group of 2-3 students: Galileoscope for the long focal length lens and 1 20-cm focal length lens assemble the eyepiece for the short focal length cm focal length lens lens in this activity. The focal lengths will be ruler different (50cm and 2cm respectively). drawing paper colored pencils or markers an assortment of tiny objects from the classroom For each student: Copy of STUDENT HANDOUT: Simple Magnifiers GO: Simple Magnifiers 1. Break the students into groups of two or three students and give each group the drawing materials and an assortment of tiny objects. 2. Have the students complete the worksheet, STUDENT HANDOUT: Simple Magnifiers. Assist the students as needed. 17

18 What s Really Happening Here When a converging lens is less than one focal length away from an object, it produces a virtual image that is right side up and magnified. Recall that virtual images cannot be projected onto a screen. You can view a virtual image by looking through the lens at the object. The converging lens in your eye will project a real image onto your retina. When an object is at the focal point of a converging lens, all the incoming light rays are parallel when they leave the lens. Therefore, an object at the focal point will not form an image. The focal point is the flip point. If the lens is closer to the object than the focal point, the image is right side up. If the lens is farther away than the focal point, the image is upside down. You might notice a similar effect with a concave mirror such as a make up mirror. Going Further 1. Have a table set up with the empty juice bottle (an empty soda bottle will work), container of water, and some small objects (i.e. coins, beads, text, etc.) Ask the students if they can make a magnifier out of the juice bottle (fill it with water). Then have them draw what they see looking through the bottle at the small objects. 2. How is the glass bottle filled with water similar and dissimilar to the lenses in the kit? 3. Can students find anything around the room or at home that could also be used as a magnifier? 18

19 STUDENT HANDOUT: Simple Magnifiers What You Need 1 20-cm focal length (thinner) lens (or Galileoscope objective lens) cm focal length (thicker) lens (or Galileoscope eyepiece) ruler drawing paper colored pencils or markers an assortment of tiny objects from the classroom What To Do 1. Look at a tiny object with your eye. Make a sketch of a couple of the objects in the space below. 2. Your teacher will give you two lenses. Examine the tiny objects through the 20-cm focal length lens. Make a sketch of the objects as they appear through the lens in the space below. 3. Examine the tiny objects through the 7.5-cm focal length lens. Make a sketch of the objects as they appear through the lens in the space below. 4. Compare views through the two lenses. Which lens produces a larger magnification? 19

20 5. Move the 7.5-cm lens closer to and farther away from the objects. What is the relationship between the distance to the objects and the magnification? 6. Looking through the 7.5-cm lens at one of the objects, slowly move it farther away from the object. Keep moving the lens farther away until you see the image flip over and become upside down. What do you think is happening at this point? 7. Try this with the 20-cm focal length lens. Does it behave in the same way as the 7.5-cm lens? 8. Measure the distance from the object to the lens at the point where the object flips over. Record your distance for each of the two lenses. What do you notice about this distance? THIS PAGE MAY BE PHOTOCOPIED FOR CLASSROOM OR WORKSHOP USE 20

21 Build a Refracting Telescope I Overview Now that we have explored what a single positive lens can do, we will investigate what happens when we use two positive lenses together. By doing so, students will, in effect, build a refracting telescope. Students Will Learn Focusing is done by adjusting the distance between the two lenses. To achieve the greatest magnification, the most curved lens (shortest focal length lens) is the one closest to the eye. The two-lens system will invert the image. What You Need: For the entire class: Colored lamp For each group of 2-3 students: 1 20-cm focal length lens (or Galileoscope objective lens) cm focal length lens (or Galileoscope eyepiece) ruler Getting Ready Place the colored lamp in a prominent position in the room where all students can easily see it. Make sure the lamp is several meters away from the closest group. GO: Build a Refracting Telescope I 1. Ask your students about instruments or devices they can use to see far away. Binoculars and telescopes are good answers (binoculars are just two telescopes placed side by side). Ask students if they have ever visited a research telescope at an observatory. Tell the students that, over the next several activities, they will learn how telescopes collect light and form images of distant objects. 2. Divide the class into groups of 2-3 students. Each group should be given two converging lenses of differing focal lengths and a ruler. 3. Have the students complete the worksheet, STUDENT HANDOUT: Build a Refracting Telescope I. Assist the students as necessary. 21

22 What s Really Happening Here Light from a source (such as our colored lamp or a star) will travel to the telescope and pass through the first (closets to the object) converging lens. This lens will create a real image of the object. This lens is also called the objective lens. The second converging lens is placed less than one focal length away from the image of the first lens. The second lens will magnify the image created by the first lens. The second lens is also known as the eyepiece and is located at the end of the telescope nearer to the eye. For refracting telescopes, the objective lens has a larger diameter and a longer focal length than the eyepiece. The function of the objective lens is to collect a lot of light; a larger diameter lens will allow in more light than one with a smaller diameter. There are several reasons that the objective has a long focal length. First, a long focal length lens is thinner and lighter. If you have a large objective lens with a short focal length, you will have a very heavy lens at the front of your telescope. Since the lens can only be supported by the edges, a large lens will sag under its own weight and not maintain its shape, leading to a blurry image. Another reason for a long focal length objective lens is to reduce chromatic aberration. Chromatic aberration occurs because short wavelength light is refracted more by a lens than long wavelength light. The result is color fringes in the image. Chromatic aberration is worse for large, short focal length lenses. Making the objective a long focal length lens reduces chromatic aberration. Even though the eyepiece has a short focal length, its diameter is small and chromatic aberration is not as severe for small diameter lenses. The magnification of a telescope can be changed by using a different eyepiece lens. Recall that a shorter focal length lens gives a larger magnification when used as a magnifying glass. Similarly, a shorter focal length lens will yield a higher magnification when used as the eyepiece of a telescope. 22

23 STUDENT HANDOUT: Build a Refracting Telescope I What You Need 1 20-cm focal length lens (or Galileoscope objective lens) cm focal length lens (or Galileoscope eyepiece) ruler What To Do 1. Hold the two lenses so that the 20-cm lens is in front of the 7.5-cm lens (in other words, so the 20-cm lens is closer to you). Look at a distant object through the lenses. Move the lenses closer together and farther apart. a. Can you create a focused, magnified image of the distant object? b. Describe what you see. 2. Now reverse the lenses hold them so that the 7.5-cm lens is closer to you. Again look at the distant object through the lenses. Move the lenses closer together and farther apart. a. Can you create a focused, magnified image of the distant object? b. Describe what you see. 3. Draw the arrangement that is best for seeing the distant object. Be sure to label your diagram. 4. The lens closer to your eye is called the eyepiece. Which lens is your eyepiece? 5. The lens farther from your eye, and closer to the object you are viewing, is called the objective. Which lens is your objective? 6. Is there a relationship between the focal lengths of the lenses and the distance between the lenses when you look at a distant object? If so, what is that relationship? 23

24 7. Repeat questions #1 and 2, but this time look at a nearby object (something about 3 feet away from you). First put the 20-cm lens closer to you. a. Can you create a focused, magnified image of the nearby object? b. Describe what you see. 8. Now reverse the lenses hold them so that the 7.5-cm lens is closer to you. Again look at the nearby object through the lenses. Move the lenses closer together and farther apart. a. Can you create a focused, magnified image of the nearby object? b. Describe what you see. 9. Which arrangement is best for seeing the nearby object? 10. With the best arrangement, again look at the nearby object and move the lenses until you get a focused image. Have your partner measure the distance between the lenses while you hold them steady. Record the distance here. 11. How does this distance change when you look at a nearby object? THIS PAGE MAY BE PHOTOCOPIED FOR CLASSROOM OR WORKSHOP USE 24

25 Build a Refracting Telescope II Overview In this activity, students will learn about the functions of the two lenses in a refracting telescope. Students will use one lens, the objective, to project an image onto a screen. The second lens, the eyepiece, will then be used to magnify the image. Students Will Learn The first lens creates an inverted, real image on the screen. The second lens acts as a simple magnifier, making the image larger. What You Need: For the class: Colored lamp For each group of 2-3 students: 1 20-cm focal length lens (or Galileoscope objective lens) cm focal length lens (or Galileoscope eyepiece) 1 velum screen 3 Styrofoam cups tape ruler Getting Ready Place the colored lamp in a prominent position in the room where all students can easily see it. Make sure the lamp is several meters away from the closest group. If you can, darken the room by turning off some lights and closing any blinds or curtains. GO: Building a Refracting Telescope II 1. Distribute the materials to each group of 2-3 students. 2. Have the students complete the worksheet, STUDENT HANDOUT: Build a Refracting Telescope II. Assist the students as needed. What s Really Happening Here Students frequently have difficulty understanding the function of the lenses in a refracting telescope. This activity attempts to make the role of each lens clear. The front lens, or objective, is placed one focal length from the screen. The lens focuses the light and creates a smaller, inverted image. The use of the screen is meant to let students see this image. The second lens, or eyepiece, is held less than one focal length away from the screen. Recall that when a lens is held less than one focal length away from an object, the image is right side up and magnified. The eyepiece magnifies the image. In other words, the object for the eyepiece is really the image formed by the objective lens. A normal 25

26 refracting telescope does not have a screen, of course. The lack of a screen makes it difficult for some students to visualize what is happening inside a telescope. 26

27 STUDENT HANDOUT: Build a Refracting Telescope II What You Need 1 20-cm focal length lens (or Galileoscope objective lens) cm focal length lens (or Galileoscope eyepiece) 1 velum screen 3 Styrofoam cups tape ruler What To Do 1. Make lens holders and a holder for the velum screen following the same procedure you used in Finding the Focal Length Using a Distant Object. 2. Recall from the last activity that the best arrangement of lenses to view a distant object was to have the shorter focal length (7.5-cm) lens as the eyepiece and the longer focal length (20-cm) lens as the objective. Use the objective to project an image of the light onto the velum screen. 3. Place the eyepiece behind the velum screen. Look through the eyepiece at the screen. Move the eyepiece until you see a focused, magnified image of the colored lamp. 4. Measure the distance from the objective to the screen. Measure the distance from the eyepiece to the screen. Is there a relationship between these distances and the focal lengths of the lenses? 5. Do you think the screen is necessary in this set up? Explain your reasoning. 6. Test your hypothesis by removing the screen and looking at the colored lamp again. Describe your observations. THIS PAGE MAY BE PHOTOCOPIED FOR CLASSROOM OR WORKSHOP USE 27

28 Build a Galileoscope Overview The Galileoscope has been created for the International Year of Astronomy (IYA) to give students the opportunity to recreate the historic observations of Galileo. The Galileoscope is an inexpensive telescope with high optical quality and is easy to use for the beginning stargazer. The telescope requires some simple tools-free assembly allowing students to see the internal elements of a refracting telescope. Students Will Learn How to assemble a small telescope The layout of the optical elements in a small telescope. How to focus and use a telescope. What You Need Galileoscopes for the class. You may wish to have each student build one or work in teams of two or three. Getting Ready Be sure to order Galileoscopes so they arrive on time. Galileoscopes can be ordered at Build a Galileoscope yourself and test it before attempting to do this activity with students. Be sure you can find the focus and can help the students find the focus as well. Go: Building a Galileoscope 1. Tell the students they are going to build a small refracting telescope similar in size to the one Galileo used to make his historic observations. 2. Warn the students that this is NOT A SOLAR TELESCOPE! IT SHOULD NEVER BE POINTED AT THE SUN! 3. Pass out the Galileoscope kits to the students. The Galileoscope come with pictorial assembly instructions. 4. As the students complete their telescopes, have them attempt to focus on a distant object such as some trees outside. Help the students focus the telescopes. It can be difficult to find the focus point the first time. The telescope focuses by sliding the eyepiece holder forward and backward. Be sure the students are moving the focuser 28

29 slowly. If they slide the focuser quickly, they may go past the focus point without realizing it. 5. Show the students how to mount the telescope on a camera tripod. It will be necessary to mount the telescope to hold it steady enough to produce a pleasing image. Troubleshooting The Galileoscope Although it is deigned to be easy to assemble and use, there are a few common mistakes that can be made when assembling and using the Galileoscope. 1. Be sure the objective lens is oriented correctly. Check its orientation against the diagram in the directions. 2. Check the orientation of the eyepiece and its lenses. The grooves should help you get the eyepiece lenses oriented correctly. 3. Move the focuser SLOWLY throughout its entire range when looking for focus. If you move it too quickly, you may pass the focus point without realizing it. If you do not try the whole range, you may miss the focus point entirely. 4. If you are trying to focus on a nearby object, the focuser usually needs to be pulled farther out. Push the focuser in to focus on distant objects. 5. Try focusing on a very distant object. The Galileoscope (as all telescopes) has a near focus point. That means you cannot focus on objects very nearby (closer than a few meters). If you are looking at objects in the room, this may be your problem. Try going outside or to a very long hallway where you can focus on objects much farther away. Going Further 1. Point out the eyepiece design of the Galileoscope to the students. Ask them how it differs from the eyepiece they used in Build a Refracting Telescope I. Ask students why they think we used four lenses instead of one. Have them research eyepieces to answer the question. 2. You may wish to explore the resolution of the telescope. A simple way to approximate resolution is by using an eye chart. Students know they should be able to read the 20 foot line from 20 feet away. How far away can they read the 20 foot line with the Galileoscope? 3. Have the students use the telescope at night. See the next section of the guide for suggestions on observing projects. 4. You can use the Galileoscope to observe the Moon during the day. Arrange a time to observe the Moon when it is visible during class. 29

30 Vendor Information Below is a list of parts and vendors we have used in the past to obtain lenses and other items for the activities outlined in this manual. Item Catalog or Part # Vendor Information Laser Level TOL10206 Surplus Computers 20 cm focal length double convex lens 7.5cm focal length double convex lens 100mm diameter 20cm focal length double convex lens # L1914D Surplus Shed 8408 Allentown Pike Blandon, PA Phone # L1912D Surplus Shed 8408 Allentown Pike Blandon, PA Phone # L2006D Surplus Shed 8408 Allentown Pike Blandon, PA Phone Velum Flat 5x7 cards (25) Mountaincow P.O. Box 2702 Providence, RI Phone Custom Matte Frames #WV101 Worldview Pictures 373 Dawson Drive Camarillo, Ca Phone Galileoscope Acrylic Block HA001 Acrylic 3x5x1/2" block Onyx Expressions Anna Green Houston, TX Phone:

31 More Background for the Interested Educator Ray Tracing How is it that we can use a lens to magnify our thumb, and in other instances we can use the same lens to invert a distant image onto a screen? How do these images form? To explain how an image is created, scientists use a model called ray tracing. Ray tracing describes the direction the light is heading. Light originates from a source. As long as light is in a uniform medium, we can think of light rays traveling in a straight line until the light encounters a boundary or barrier of some type. Ray tracing can be used to find where the image forms when light rays from an object pass through an optical system consisting of one or more lenses or mirrors. Recall the activity, Finding the Focal Length Using a Distant Object, where we used a very distant object to create an image. When an object is very far away from the lens (much farther than the focal length), we consider the object to be infinitely far away. When you are very far away from the object, the light rays are spreading out so slowly they are difficult to distinguish from parallel light rays. A converging lens will focus all incoming parallel light rays to the focal point. 31

32 Ray Diagrams Now with this basic understanding we can start to construct a ray diagram of our optical system. Step One: First draw the lens. Next draw a line through the center of the lens. This line is called the optical axis. Step Two: Draw in the object and image locations and the location of the focal point. Make sure that you draw everything to scale. Step Three: Draw a ray parallel to the optical axis from the top of the object to where it meets the center of the lens. (Note: To simplify things, the lens can be drawn as a single vertical line.) This ray will go form the center of the lens, through the focal point, to the top of the image. 32

33 Step Four: Draw a line from the top of the object through the center of the lens to the top of the image. This is one of the most straightforward ways to create a ray diagram for a positive lens. This method can be used to predict where an image is going to be. It also can be used to predict how big the image is going to be. Alternatively, if you know where the image is, you can find how big and how far away the object was. How Light Travels Light is composed of electromagnetic waves. These waves travel in a straight line that can be though of as a ray. A ray has an origin and direction. This ray of light will travel in a straight line until its path crosses into a different medium. Once the light ray enters the other medium, it is bent into another direction (either away from or closer to the surface normal). The amount that the ray of light is bent is determined by Snell s Law: n1 sinθ1 = n2 sinθ2 The subscripts denote which medium the light is coming from (medium 1) and is entering (medium 2). The angle θ x is the angle that the ray makes with the surface normal within the given medium. The index of refraction of the medium is designated n x. The index of refraction is the ratio of the speed of light in vacuum to the speed of light in the medium: c n = v The letter c is the speed of light in vacuum (3*10 8 m/s) and the letter v is the speed of light in the medium. Common values for n are: n = 1 for vacuum and air n = for water n 1.5 for glass 33

34 Lenses When you look at a very distant object (very distant is defined as the distance to the object is very large when compared to the focal length of the lens) an image will form at the focal point. This image will be upside down and much smaller than the object. A camera works based on this principle. A converging lens focuses an image on the film. There are many different types of lenses, from single pieces of glass called singlets to complex multiple-element lenses found in expensive cameras. We use singlets in this module. Singlets can either have positive or negative power. Negative singlets can be used with positive powered lenses to create types of telescopes and other such devices. Positive Singlet Positive lenses are lenses that are thicker in the middle than on their edges. Their shape is what causes the light to bend and come to a focus. This module deals only with positive lenses. Positive lenses are used to create images. Light comes from an object and passes through the lens to form an image (see Going Further: Ray Tracing and Diagrams ). There are two types of images that can be formed with positive lenses. A real image is one that can be projected onto a surface. A real image and its original object are always on opposite sides of the lens. An example of a real image is the image created by an overhead projector. In contrast, a virtual image is one that cannot be focused onto a screen. It can only be seen by looking back at the object through the lens. An example of a virtual image is when you look through a magnifying glass. Lenses are designed using the following equation called the Thin Lens Formula: O I = 1 F where O is the distance from the object to the lens, I is the distance from the lens to the image, and F is the focal length of the lens. 34

35 There are two common methods to measure the focal length of a positive lens. The simplest method is the object at infinity method. If you use an object that is very far away (many times longer than the focal length such as a distant building, light bulb, etc.), O becomes very large, so 1/O becomes very small. Assuming 1/O very close to zero, the equation above becomes I = F. In other words, the image forms at the focal point. Two positive lenses can be put together to create refracting telescope. It is called a refracting telescope because it is made of lenses that bend or refract the light. There are two lenses used in the refracting telescope contained in the module. The lens that is held closest to the eye is called the eyepiece and the lens that is farthest away is called the objective (or objective lens). The objective creates a real image (see Building a Refracting Telescope II ). This real image then becomes the object for the eyepiece and the eyepiece acts as a magnifier. Lenses suffer from a problem called chromatic aberration. The index of refraction varies slightly for different colors of light. The index of refraction is slightly higher for blue light than for red light. Dispersion is the name given to the process of a lens spreading out different colors of light. If you try to focus and object with a single lens, you will see some color fringes. This effect is particularly noticeable for larger lenses and at higher magnification. Therefore, it is not a significant problem in the human eye. Compound Lens: Chromatic aberration can be corrected by using compound lenses. Compound lenses consist of two or more singlets. The singlets can have air, oil or other substances in between them to help achieve the desired focal length and correct chromatic aberration. The problem can be further reduced by using special low dispersion glass. Modern glass containing fluorite is frequently used in high quality refractor telescopes due to its low dispersion. 35

36 Lenses are not the only optical component that can create real images. A concave mirror can create real images as well. Like the positive lens, the concave mirror reflects light in such a way that it can come to a focus at a focal plane. Concave mirrors are important because they are used in many types of reflecting telescopes. Diverging (Negative) Lenses All of the lenses used in this module are converging lenses and have a positive focal length. It is also possible for lenses to spread light out. These lenses are called diverging lenses. Diverging lenses are concave. Concave lenses take light rays that are parallel and spread them out. The rays spread out so they appear to be coming from a common point. The focal point of a diverging lens is on the same side of the lens as the light source. The focal length of a diverging lens is negative since it is on the opposite side of the lens as the focal point of a converging lens. Eyeglasses Most people s eyeglasses are negative lenses, as opposed to the positive lenses used in this module. Therefore using your or a student s glasses (those which correct for nearsightedness or farsightedness) for the experiments in this module will not usually work. For those individuals who have reading glasses, their glasses can be used in the experiments in the module. Reading glasses are composed of positive lenses. The reason that people have glasses is that their eye either focuses light in front of or behind the retina. The retina is responsible for taking the image that is focused on it and turning it into an electrical signal that our brain can interpret. People who have images focusing in front of the retina are said to be nearsighted and have the condition called myopia. People who have images in focus behind the retina are said to be farsighted and have the condition called hyperopia. Most people who have eyesight problems are myopic; the condition results in blurry images. Negative lenses are used to correct myopia and positive lenses are used to correct hyperopia. One way to check to see if you are myopic or hyperopic is to take off your glasses, hold them in front of you and look at a distant object. If that object appears bigger through your glasses than without you have hyperopia; if that object appears smaller you have myopia. Another affliction that can affect a person s eyesight is astigmatism. Astigmatism occurs when the lens in the eye is not completely circularly symmetric, meaning that the curvature of the lens from top to bottom is different than the curvature from left to right. 36